1. Field of the Invention
The present invention relates to a method for the synchronization of induction coils supplied by different power converters and capable of acting on a common heating zone of an induction heating system, particularly an induction cooking hob, or capable of acting on different adjacent heating zones. The present invention relates as well to an induction heating system, particularly an induction cooking hob which can carry out the above method. In the above kind of induction heating systems it is necessary to synchronize induction coils, powered by different power converters, whose generated electromagnetic (EM) fields are mutually linked due to coils proximity or due to dual zone configuration (for instance two coils concatenated: an inner coil and outer coil). This configuration is highly problematic because the interference between the coils can generate vibrations in the range of audible frequencies, highly unpleasant for the user.
By synchronizing the coils, i.e. by synchronizing the switching of the power transistors of the power converters supplying the coils, the generation of noise during normal operation can be prevented, and the comfort of the user is guaranteed.
The present invention relates particularly to all the configurations where the load (i.e. a metal cooking vessel) receives power from multiple induction coils, and such coils are powered by different power converters and the EM fields generated by at least two coils are mutually linked.
The attached FIG. 1 shows some example of configurations of load heated by multiple coils.
2. Description of the Related Art
An induction heating system consists of a source of alternating electromagnetic flux coupled with a load, typically a ferromagnetic vessel. Eddy currents induced in the bottom of the vessel are dissipated for Joule effect, becoming heat that warms up the coil.
The electromagnetic coupling involves mechanical vibrations of the vessel. These vibrations have the same frequency as the electromagnetic flux: actual frequency ranges adopted in induction hobs are always above the audible range, therefore there is no generation of acoustic noise for the user.
If multiple sources of alternating electromagnetic flux are coupled with the same vessel, the use of frequencies above the audible range is not enough to ensure the absence of acoustic noise. If two sources are not synchronized, the superposition of the two waves on the vessel generates interference waves with different frequencies, which can fall into the audible range.
A typical interference is the beat, i.e. an interference whose frequency is the difference between the frequencies f1 and f2 of the two main signals. If two waves y1 and y2 have slightly different frequencies, or they're not synchronized, their superposition y1+y2 generates a beat with a period Tbeat much greater than the periods of y1 and y2, as shown in FIG. 2.
For avoiding the problem, real applications must guarantee the synchronization of all the sources (coils) coupled with the same load (vessel). If such sources are powered by the same power converter the problem is quite simple: being the frequency generator (typically a microcontroller with a clock generator) the same for all the sources, it is enough that the coils are supplied at the same frequency at every time.
If the coils are powered by different power converters (with different frequency generators) the application of the same frequency is not enough: being the frequency generators independent, they are not synchronized, unless a synchronization mechanism is provided.
The problem of the acoustic noise due to beat generation, highly probable when multiple coils, supplied by different power converters, are coupled with the same load, is well known in the art of induction heating system, particularly for their applications to cooking appliances.
A solution is shown in WO-A-2007/048700 which discloses a direct link between the power converters, i.e. a cable that connects the microcontrollers of the converters, or their clock sources, or the control signals of the power transistors, and such cables send a signal from a first power converter (master) to the other (slave) that serves as a synchronization for the generation of the pulse width modulation driving signals for converters.
This known solution presents some drawbacks since the connection between microcontrollers implies the use of shielded cables and it increases the overall cost of the appliance. Moreover such known solution implies also a direct electrical link between the two power converters, which can create problems of reliability and electrical insulation.
Another solution is known from US-A-2007/0135037 which discloses a way to operate two induction coils jointly by means of one converter circuit with two outputs, each of the output being connected to an induction coil. The converter circuit is operated in such a way that the frequency of the noise generated by the superposition of the two different switching frequencies is outside the human audibility range of frequencies. This solution makes the circuit more complex and somehow limits the possibility to change the switching frequencies of the converters in a sufficiently wide range.